The rapid growth in the diversity of communication networks and the number of users of such networks is increasing the number of instances where two vocoders are placed in tandem to serve a single connection. The name "vocoder" stems from the fact that the vocoder is a codec having applications specific to encoding and decoding, i.e. compressing and decompressing, voice signals. Vocoders provide speech compression of a digitized voice signals as well as the reverse transformation. Typically, a voice signal is digitized through one of many quantization techniques. Examples of these techniques are Pulse Amplitude Modulation (PAM), Pulse Code Modulation (PCM) and Delta Modulation. For purposes of this description we will refer to PCM as the input format for the vocoder although no limitation to this format is intended for use by the present invention. A vocoder includes an encoder stage that will accept as input a digitized voice signal and output a compressed signal, a possible compression ratio being 8:1. As for the reverse transformation, the vocoder is provided with a decoder stage that accepts the compressed speech signal and outputs a digitized signal, such as PCM samples.
In many wireless communication networks, a first vocoder integrated in a mobile station is used to compress the speech of a first mobile user. The compressed speech is transmitted by Radio Frequency (RF) equipment to a base station controller (BSC) and a mobile switching center (MSC) serving the local mobile station. Either the BSC or the MSC has a second vocoder, depending on the particular wireless system i.e. TDMA, GSM etc., which decompresses the received compressed signal into PCM samples. The PCM samples are transmitted over a digital trunk of the network, such as a public switched telephone network (PSTN), to a second MSC and BSC serving a second mobile station. A third vocoder at the second BSC or MSC then recompresses the PCM samples for RF transmission to the second mobile station. A fourth vocoder integrated into the second mobile station then decompresses the received compressed speech signal to synthesize the original speech signal from the first mobile station. Such an arrangement of multiple vocoders is commonly referred to as tandemed vocoders which serve a single connection. A specific example of tandemed vocoders may involve a call from a wireless terminal operating according to the North American Time Division Multiplexing Access (TDMA) system to a European Standard Global System for Mobile (GSM) mobile phone.
The main advantage of compressing speech for RF transmission is that it uses less of the limited available RF channel bandwidth for transmission, while the main disadvantage is the loss of speech quality. Operating tandemed vocoders significantly degrades the voice quality of speech, thus providing the desire to limit the number of times that speech is compressed and decompressed in a single connection i.e. between two mobile users.
As disclosed in commonly assigned international application PCT 95CA704 there is disclosed a method to eliminate the condition of tandemed vocoders through a method called tandem free operation (TFO). TFO is defined as a codec bypass action if the vocoders in both mobile stations are the same, e.g. both GSM vocoders. TFO is also defined as using a common format across the transport network if the vocoders at each terminal are different e.g. GSM-CDMA. The basic idea behind this approach is the provision of a digital signal processor including a vocoder, and a bypass mechanism that is invoked when the incoming signal is in a format compatible with a downstream vocoder. Through signaling and control, the digital signal processor associated with the first BSC or MSC serving the first mobile station determines that a compatible vocoder resides at a second BSC or MSC serving a second mobile station. In such a case, the first digital signal processor associated with the first BSC or MSC, rather than converting the compressed speech signals into digital samples, i.e. using a PCM format, invokes the bypass mechanism and outputs the compressed speech to the transport network. The second digital signal processor associated with the second BSC or MSC receives the compressed speech from the transport network and also invokes the bypass mechanism. Compression of the digitized speech signal occurs only once, at the first mobile station, and decompression of the compressed speech signal occurs only once, at the second mobile station. The contents of this international application are incorporated herein by reference.
There is desired an improved mechanism for identifying the capability of wireless network equipment to facilitate TFO in a wireless communication system such as to improve voice quality.